1. Field of the Invention
The present invention relates to power supplies employing inductive systems and high frequency switches to regulate an output voltage and to improve the power factor.
2. Description of Related Art
Alternating current from a power line can be rectified through a full wave bridge to charge a capacitor. This simple rectifier will draw bursts of current around the voltage peak of the power line. This distorts the line current, creating high instantaneous demands and harmonic distortion. The line current can be further disturbed by reactive loads or other factors, which cause a phase shift between the current and voltage on the power line. The disparity between the line voltage and current is disadvantageous and much work has been done in this field under the heading of power factor correction.
Known voltage doubler circuits use a serially connected pair of capacitors whose common terminal is connected to one side of a power line. The other side of the power line connects through oppositely poled rectifiers to the non-common terminals of the different capacitors. Constructed in this simple fashion, a voltage doubler has no regulation and also draws current pulses in a way that will degrade its power factor.
The prior art power supply of FIG. 1 feeds a full wave bridge 12 through an electromagnetic interference filter 10. The rest of the circuit is an "up-pulser." That circuit is essentially a boost converter having an inductor L1 serially connected with a rectifier CR1 to charge output capacitor C1 at output terminals T1, T2. A high frequency switching transistor Q1 is connected from the junction of inductor L1 and diode CR1 through a series resistor R1 to output terminal T2. When the transistor Q1 conducts, current flows from the full wave bridge through inductor L1 to return terminal T2, to build a magnetic field in inductor L1. When transistor Q1 later stops conducting, the voltage across inductor L1 reverses and the current through inductor L1 continues to flow in the same direction, but now through rectifier CR1 to charge capacitor C1. Capacitor C1 is charged to a higher potential than the peak voltage from the full wave bridge 12.
Significantly, the current through inductor L1 is uninterrupted. The current waveform I.sub.s ramps up and down stepwise (for simplicity, an uncharacteristically small number of steps are shown). Thus, the current waveform I.sub.s will have small, high frequency perturbations that can be easily removed by filtering. This scheme avoids the high energy harmonics caused by periodically interrupting the current to drop it from a peak value to zero.
A disadvantage with this configuration is that terminals T1 and T2 do not constitute stable reference potentials. For example, terminal T2 is connected through bridge 12 alternatively, to either the high or the low line of the power main. This changing reference potential tends to introduce electromagnetic interference with any load connected to terminals T1 and T2. In addition, in systems employing three phase power, the power supply of FIG. 1 cannot be simply replicated three times, because their outputs cannot be tied directly together without isolating transformers or the like.
See also U.S. Pat. No. 3,215,925 for an early implementation of this type of system. Similar systems are shown in U.S. Pat. Nos. 4,677,366 and 4,940,929. Other power circuits of lesser relevance are shown in U.S. Pat. Nos. 2,999,970; 3,223,915; 3,284,692; 3,746,967; 3,796,941; 3,845,374; 3,889,176; 4,224,662; 4,471,855; 4,672,526; and 5,119,283.
Control circuit 14 of FIG. 1 may employ known integrated circuits. For example, a Unitrode high power factor preregulator, model number UC1854, can produce a chopping frequency of, for example, 55 kHz. This known circuit can respond to various feedback signals: the instantaneous current and voltage from full wave bridge 12, the output voltage across capacitor C1, and the RMS value of the voltage from bridge 12 (estimated with a second order filter, for example). This system compares the voltage across capacitor C1 to an internal standard and then this difference is (a) is multiplied by the instantaneous output voltage of bridge 12 (b) and divided by the square of the RMS value of that voltage. This factor is then compared in an error loop to the instantaneous current from bridge 12 to regulate the duty cycle of transistor Q1.
The output voltage across capacitor C1 is regulated by regulating the average duty cycle of transistor Q1. The instantaneous bridge current is regulated by regulating the instantaneous duty cycle of transistor switch Q1. By adjusting this instantaneous duty cycle, the magnitude of current and voltage at the power main can be kept in phase and, for constant loads, proportional. This relationship avoids current surges and harmonics and improves the effective power factor of the supply as seen by the power lines.
U.S. Pat. No. 4,831,508 shows in FIG. 12 a boost regulator employing an inductor carrying bidirectional current and a bidirectional shunt switch, both feeding a full wave bridge to charge a pair of serially connected capacitors. The center tap between the capacitors is connected either directly to neutral or to the center tap of a transformer connected across the power line.
The shunt switch operates at a low frequency with a switching cycle that repeats only once per half cycle of the power line. Thus this system will produce relatively high harmonic components. Also, because the output voltage is supplied from a full wave bridge, the high and low output terminals will be alternately referred to neutral. Thus this output will have a relatively unstable high potential, which tends to produce electromagnetic interference, and to impede direct connection (without isolation) when multiple phases are employed.
U.S. Pat. No. 5,224,025 shows in FIG. 1 a full wave bridge charging a capacitor through a serially connected inductor and rectifier whose junction is shunted through a transistor switch. This capacitor is not the power output and this configuration has many of the disadvantages noted above.
Known systems have employed the six step, pulse width modulation (PWM) control technique (also known as space vector modulation). For example, a three phase PWM boost rectifier can employ three inductors separately connected to each phase of a power main. The output terminal of each inductor is connected by two switches to either end of a common output capacitor. Each switch has a shunting rectifier connected in parallel across it. See V. Vlatkovic, D. Borojevic, F. C. Lee, C. Cuadros, and S. Gataric, A New Zero-Voltage Transition, Three Phase PWM Rectifier/Inverter Circuit, Virginia Power Electronics Center (published 9/93, but not necessarily prior art). For a three-phase buck rectifier, see S. Hiti, V. Vlatkovic, D. Borojevic, and F. C. Lee, A New Control Algorithm for Three-Phase PWM Buck Rectifier with Input Displacement Factor Compensation, Virginia Power Electronics Center (published 9/93, but not necessarily prior art).
Accordingly, there is a need for an improved rectifier for supplying regulated voltage from single or multiple phase power lines while shaping the line current waveform to improve the power factor and to reduce harmonic distortion.